Out of a total of 906 submarine accidents from late 2004 through 2013, according to Naval Safety Center data, only 18 were considered "Class A" — the most severe, in which someone was killed or permanently disabled, or there were damages of $2 million or more.

Four sailors aboard the Harry S. Truman were injured Tuesday when a line parted during a refueling between the Norfolk-based Truman and the Military Sealift Command fleet oiler Patuxent off the coast of Florida.

On Oct. 13, 2012, the Norfolk-based aircraft carrier Harry S. Truman was conducting training off the coast of Florida ahead of a deployment. Three smaller Navy vessels from Norfolk were there, too: the destroyer Gravely, the cruiser San Jacinto and the submarine Montpelier.

The plan was for what the Navy calls an anti-submarine warfare exercise. The Montpelier, playing an enemy "red sub," was to ascend almost to the surface to surveil the Truman, then submerge and approach for an attack. The Gravely and the San Jacinto were tasked with protecting the carrier.

Things did not go as planned.

At about 3:30 that afternoon, the Montpelier slammed into the San Jacinto with so much force that the sub's rudder detached. The Montpelier dove, and for more than an hour, Navy officials couldn't find or make contact with the vessel.

A newly released Navy investigation, made public more than a year after media outlets requested it, provides a revealing look at the collision of two ships — a cardinal sin in the Navy.

The report places blame squarely with the Montpelier's watch standers, including the sub's then-commanding officer, Cmdr. Thomas Winter, who made a key misjudgment about the San Jacinto's location. Winter was later relieved of duty.

But the report goes further, concluding that officers above him failed in planning the exercise. While leaders aboard the surface ships thought the exercise involved a "benign, slow-moving sub," the report says, the Montpelier thought it was supposed to "conduct simulated attacks, and thus portray an aggressive participant.

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The exercise was intended to be basic, but what played out was a complex "free-play" war game involving a fast-moving sub operating at near-surface depths close to other ships — a rare scenario that went beyond the training level of some of the sailors involved, the report says.

It says that with clearer objectives for the exercise and stronger leadership, the misunderstandings could have been avoided, and if planners had followed risk management protocols, they would have seen the potential for a collision and taken precautions.

The rear admiral who led the investigation, Ann Phillips, apparently thought the two top officers overseeing the exercise — Rear Adm. Kevin Sweeney, the head of the Truman strike group, and Capt. John Fuller, then-head of Destroyer Squadron 22 — were negligent.

But Norfolk's top admiral, Bill Gortney, the four-star head of U.S. Fleet Forces Command, overrode Phillips. Her findings regarding Sweeney and Fuller were redacted from the version of the report given to the media.

"Although (they) failed to employ rigid operational risk management in the planning and execution of the subject anti-submarine warfare exercise, their actions did not constitute negligence," Gortney wrote in his endorsement of the investigation. "I will personally discuss the events of 13 October 2012, and the results of this investigation with both (of them)."

Through a Fleet Forces spokeswoman, Gortney declined to discuss his decision.

He wrote in his endorsement that he bears some responsibility: "U.S. Fleet Forces Command is at the top of this command and control, and as such, the failure in the chain of command to properly plan and execute this exercise rests firmly with this endorser."

In addition to Winter, six Montpelier sailors were formally disciplined after the collision, which caused no fatalities, said the Fleet Forces spokeswoman, Lt. Cmdr. Reann Mommsen.

Repairs have cost $81 million so far, Mommsen said. Of that, $70 million was for the Montpelier.

In her recommendations, Phillips called for training improvements and a comprehensive review of the preparation process for exercises in which surface ships and subs interact.

The Navy has since done so, said Rear Adm. Pete Gumataotao, the head of Naval Surface Force Atlantic. "We have updated our procedures," he said in a statement to The Virginian-Pilot, "and have also begun completing more basic exercises together before we conduct a full group sail."

The investigation included interviews with more than 100 people and a re-creation of the crash by physicists at Johns Hopkins University.

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The report recounts what happened:

While the Truman was busy launching aircraft, the Gravely and the San Jacinto were moving with the carrier, in protective "screen positions." About an hour into the exercise, which was supposed to last six hours, the San Jacinto left the Truman. After a helicopter landed on its deck, the San Jacinto was ordered to hurry back.

The Montpelier, which had been conducting surveillance close to the surface, descended to pass stealthily underneath the Gravely. The sub then ascended almost to the surface — called periscope depth — closer to the carrier.

It was then that Winter and the Montpelier's watch standers made a key mistake: They failed to detect that the San Jacinto had made a 180-degree turn; they thought it was moving farther away, but it was on a constant bearing, getting closer.

Aboard the San Jacinto, a lookout spotted the Montpelier's periscope off the bow, a mere 100 yards away. He shouted a warning.

"Sub, dead ahead!" another sailor yelled.

"All back emergency!" the San Jacinto's officer of the deck ordered.

It was too late. "He knew instantly that they were going to hit," the investigation says of the officer of the deck. "It was just a matter of where."

Aboard the Montpelier, Winter was manning the periscope. When he finally saw the San Jacinto, he ordered the sub to go deep, and then he heard it — "a god-awful metal on metal sound," he later told investigators.

The San Jacinto's officer of the deck told investigators that if his ship hadn't slowed, the vessels would have collided head-on.

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Winter could not be reached for comment. Investigators faulted him for failing to follow standard procedures for operating at periscope depth, and for the Montpelier's aggressive stance. "Be ready to aggressively engage the enemy," he'd instructed the crew the night before, according to the investigation. "We are warriors and in this exercise we get a chance to practice our craft."

Winter had been in the Navy almost 20 years at the time of the collision, and his record was otherwise strong, the investigation says. It acknowledges that the Montpelier's executive officer and top enlisted sailor had been aboard for about a week before the collision, and weren't yet providing the "forceful backup" Winter needed. He'd been "placed in a situation where his personal intervention was required to run many aspects of his ship," the report says.

It notes that the Montpelier's sonar chief petty officer, a key player in detecting nearby ships, had slept for "only a couple of hours in the 24 hours prior to the collision."

In his review of the investigation, the head of Submarine Force Atlantic, Vice Adm. Michael J. Connor, wrote that the collision brought to light critical culture problems. The Montpelier wasn't following long-standing submarine safety practices, Connor said, including the concepts of forceful backup and "defense-in-depth."

Across the force, he wrote, there was a migration away from a building-block approach to training that asks sailors to crawl, then walk, then run. He pledged to address "the unnoticed erosion of safety standards over time."